Sealing arrangements for downhole tools which have a component which moves relative to a housing, for example rotary or reciprocating shafts, are known in the industry. A typical example is described in U.S. Pat. No. 5,639,227, Seal arrangement for the drivehead of a downhole rotary pump.
One limitation of the current state of the art is that these rotary or reciprocating seals can wear out, due to the fact that the surface of the seal is in continuous contact with the shaft. That is, the seal is continually energised or engaged, with an engagement force between the seal surface and the shaft surface. Various surface treatments, lubricants and surface finishes all help to reduce the wear of the seal, extending the useful life of the product, but only to a limited extent. In certain applications, the necessity of the seal to perform a sealing function in a downhole tool may be only a proportion of the time the tool is in operation. In the case of the rotary or reciprocating shaft seal, the duty period may only be 5% of the operational time. However, with the current state of the art the seal will be in contact with the shaft 100% of the time, leading to limited operational life.